Marisol Manzano

Modeling of non-isothermal multi-component reactive transport in field scale porous media flow systems

Abstract A general 2-D finite element multi-component reactive transport code, TRANQUI, was developed, using a sequential iteration approach (SIA). It is well suited to deal with complex real-world thermo-hydro-geochemical problems for single-phase variably water saturated porous media flow systems. The model considers a wide range of hydrological and thermodynamic as well as chemical processes such as aqueous complexation, acid-base, redox, mineral dissolution/precipitation, gas dissolution/ex-solution, ion exchange and adsorption via surface complexation. Under unsaturated conditions only water flow is considered, although gas pressures are allowed to vary in space in a depth-dependent manner specified by the user. In addition to the fully iterative sequential approach (SIA), a sequential non-iterative approach (SNIA), in which transport and chemistry are de-coupled, was implemented and tested. The accuracy and numerical performance of SIA and SNIA have been compared using several test cases. The accuracy of SNIA depends on space and time discretization as well as on the nature of the chemical reactions. The capability of the code to model a real case study in the field is illustrated by its application to the modeling of the hydrochemical evolution of the Llobregat Delta aquitard in northeastern Spain over the last 3500 years during when fresh-water flow from a lower aquifer displaced the native saline aquitard waters. Manzano and Custodio carried out a reactive transport model of this case study by using the PHREEQM code and considering water flow, aqueous complexation, cation exchange and calcite dissolution. Their results compare favorably well with measured porewater chemical data, except for some of the cations. Our code is not only able to reproduce the results of previous numerical models, but leads to computed concentrations which are closer to measured data mainly because our model takes into consideration redox processes in addition to the processes mentioned above. A number of sensitivity runs were performed with TRANQUI in order to analyze the effect of errors and uncertainties on cation selectivities.

The impact of the Aznalcóllar mine tailing spill on groundwater

As a consequence of a mine tailing dam collapse on the 25th April 1998, more than 4000 ha of the Guadiamar riverflat and farmlands were flooded by 4 hm3 of sulphide slurry. A number of open wells (12 of the 47 analyzed) were also flooded and the water was contaminated. Before the spill, the groundwater in the aquifers was of calcium-carbonate and calcium-sulphate type, with moderate mineralisation and near neutral pH. With the exception of some of the wells close to the mine, this groundwater had a low concentration of the metals associated with the Aznalcóllar mine. After the flood the following metals had anomalous concentrations in well water: Zn, Mn, Pb, Co, Cd and Tl. Of these, Zn seems to be the best tracer of the contamination, owing to its high concentrations. During the 5 months following the spill, water from the unflooded wells did not show an increase in metal concentration. Apart from some exceptions in August, the metal concentration in the affected wells showed a progressive decrease reaching levels closer to those in the wells free from contamination. Nevertheless, in the following dry seasons the draw-down of the water level may lead to exposure and weathering of sulphides in the wells, which could cause an increase in pollution. Therefore, thorough cleaning of all highly contaminated wells is strongly recommended.

Groundwater intensive use and mining in south-eastern peninsular Spain: Hydrogeological, economic and social aspects

Intensive groundwater development is a common circumstance in semiarid and arid areas. Often abstraction exceeds recharge, thus continuously depleting reserves. There is groundwater mining when the recovery of aquifer reserves needs more than 50years. The MASE project has been carried out to compile what is known about Spain and specifically about the south-eastern Iberian Peninsula and the Canary Islands. The objective was the synthetic analysis of available data on the hydrological, economic, managerial, social, and ethical aspects of groundwater mining. Since the mid-20th century, intensive use of groundwater in south-eastern Spain allowed extending and securing the areas with traditional surface water irrigation of cash crops and their extension to former dry lands, taking advantage of good soils and climate. This fostered a huge economic and social development. Intensive agriculture is a main activity, although tourism plays currently an increasing economic role in the coasts. Many aquifers are relatively high yielding small carbonate units where the total groundwater level drawdown may currently exceed 300m. Groundwater storage depletion is estimated about 15km(3). This volume is close to the total contribution of the Tagus-Segura water transfer, but without large investments paid for with public funds. Seawater desalination complements urban supply and part of cash crop cultivation. Reclaimed urban waste water is used for irrigation. Groundwater mining produces benefits but associated to sometimes serious economic, administrative, legal and environmental problems. The use of an exhaustible vital resource raises ethical concerns. It cannot continue under the current legal conditions. A progressive change of water use paradigm is the way out, but this is not in the mind of most water managers and politicians. The positive and negative results observed in south-eastern Spain may help to analyse other areas under similar hydrogeological conditions in a less advanced stage of water use evolution.

Chapter 5.2:Groundwater Quality Background Levels

Groundwater forms complex, three-dimensional bodies in which recharge, flow conditions and interaction with the solid matrix are point dependent. This means that, in a given groundwater body, the chemical, radiochemical and biochemical characteristics of water vary both in space (horizontally and ve...

Integrating soil water and tracer balances, numerical modelling and GIS tools to estimate regional groundwater recharge: Application to the Alcadozo Aquifer System (SE Spain)

Groundwater recharge is one of the key variables for aquifer management and also one of the most difficult to be evaluated with acceptable accuracy. This is especially relevant in semiarid areas, where the processes involved in recharge are widely variable. Uncertainty should be estimated to know how reliable recharge estimations are. Groundwater recharge has been calculated in the Alcadozo Aquifer System, under steady state conditions, at regional (aquifer) and sub-regional (spring catchment) scales applying different methods. The regional distribution of long-term average recharge values has been estimated with the chloride mass balance method using data from four rain stations and 40 groundwater samples covering almost the whole aquifer surface. A remarkable spatial variability has been found. Average annual recharge rates ranges from 20 to 243mmyear(-1) across the aquifer, with an estimated coefficient of variation between 0.16 and 0.38. The average recharge/precipitation ratio decreases from 34% in the NW to 6% in the SE, following the topographic slope. At spring-catchment scale, recharge has been estimated by modelling the soil water balance with the code Visual Balan 2.0. The results, calibrated with discharge data of the two main springs Liétor and Ayna, are 35.5 and 50mmyear(-1) respectively, with estimated coefficients of variation of 0.49 and 0.36. A sensitivity analysis showed that soil parameters influence the most the uncertainty of recharge estimations. Recharge values estimated with both methods and at two temporal and spatial scales are consistent, considering the regional variability obtained with the chloride method and the respective confidence intervals. Evaluating the uncertainties of each method eased to compare their relative results and to check their agreement, which provided confidence to the values obtained. Thus, the use of independent methods together with their uncertainties is strongly recommended to constrain the magnitude and to provide reliance to recharge estimations.